Formation of quartz silt during humid tropical weathering of dune sands

Quartz dune-sand grains in northeast Australia break down in situ under humid tropical weathering conditions to form substantial amounts of silt. Fragmentation appears to occur primarily due to silica solution along microfractures and dislocation structures in grains which have experienced tectonic deformation prior to deposition. The surface textures of quartz grains viewed with the SEM exhibit a close dependency on crystallographic properties. In the B and C horizons of some weathered late Pleistocene dunes silt forms up to 10% of the bulk sediment. Local remobilisation of such deposits by wind allows selective removal of the silt which is later deposited as a thin surficial sheet in more sheltered areas downwind. These observations provide further evidence that weathering processes provide a major mechanism of silt formation and are an important factor to be considered in relation to the origin of loess and siltstones.     

Anatomy of the middle ordovician sevier shale basin,eastern Tennessee

The Sevier Shale basin in eastern Tennessee comprises one of the thickest clastic sequences (nearly 2500 m) of Middle Ordovician age in North America. The lower one-half of the sequence is composed of Lenoir, Whitesburg, Blockhouse and Sevier Formations, in ascending order. The sequence ranges in age from Whiterockian to lower Wilderness in North American stages.The Middle Ordovician sequence exhibits tidal flat (Mosheim Member of Lenoir Fm.), subtidal (main body of Lenoir Fm.), slope (Whitesburg Fm.), anoxic basin (Blockhouse Fm), turbidite and contourite (Sevier Fm.) facies. The Sevier basin evolved in five stages: First, a widespread marine transgression initiated carbonate-shelf deposition in the study area. Second, a major tectonic downwarping event caused the stable shelf to break and subside rapidly at a rate of 60–65 cm 1000 yrs^?1, and areas of shelf facies became areas of slope and basin facies. Third, global transgressions maintained the deep anoxic conditions for nearly 10 Ma. Fourth, turbidites began to fill the basin from a westward-prograding submarine fan system. Fifth, contour currents reworked the turbidites and progressively ventilated the Sevier basin. The basin-filling process terminated with shallow-water/subaerial clastics at the end of Middle Ordovician.     

A Comparison of Office-Derived vs. Field-Derived Water Table Maps for a Sandy Unconfined Aquifer

This study compares the accuracy of two types of water table maps both of which were constructed with the object of optimizing future mapping efforts in similar environments. The. first type of map is based solely on office information, with no field verification. The second type of map is based on careful field mapping using numerous measurement points.
The office-derived maps were based on topography, surface water features, existing reports, maps and data in the files of the Wisconsin Geological and Natural History Survey; the data were not field-verified. The field-derived maps used a dense network of 236 piezometers at 176 sites in an area of approximately 170 square miles. The field project was much more expensive and labor-intensive than was the construction of office-derived maps for the same area.
The two methods produce water table maps which agree to an appreciable extent, the greatest agreement being in areas having ground water-fed streams. Differences in water table elevations indicated by the two methods range from negligible to approximately 5 feet. Thus, depending upon the availability of existing information, relatively accurate water table elevations can be delineated in similar sandy unconfined aquifers without time-consuming and expensive field work that drilling and piezometer installation entails.
Preliminary construction of office-derived water table maps enables researchers to use their resources efficiently. In some situations, expensive installation of wells and piezometers for a regional monitoring network may add little accuracy to the regional map. For localized problems, collection of additional field data will always be necessary, but can be guided by the office-derived maps. The authors caution that this technique may only be applicable to sandy, unconfined aquifers in humid climates.     

Selected Apollo 17 soils: Mineralogy and geochemistry of opaque and non-opaque phases

Soil samples 74220 (“orange soil”), 74241 and 75081 were sized and the compositions of the opaque and silicate phases were determined. The ilmenites, particularly in 74241, contain up to 7.8 wt.% MgO and display higher bireflectance than low-Mg ilmenites. They commonly contain exsolution-like chromite and rutile and occasionally are in association with native Fe in an assemblage probably resulting from reduction. The chromian ulvöspinels are similar to Apollo 11 spinels in that they contain near-equal amounts of chromite and ulvöspinel molecules. No primary chromites were observed. Most native Fe has No and Co contents of < 1 wt.%; some in 74220 contained 5–6% Ni and < 1% Co in association with schreibersite. All armalcolites examined are optically and chemically similar and the common mantling by Mg ilmenite leads us to conclude they are the ortho-armalcolite of S.E. Haggerty. We have collected optical (reflectivety and bireflectance) and microhardness data for the ilmenites, ulvöspinels and armalcolites. These are the first microhardness determinations for the last two minerals.     

The Ekman boundary-layer stress due to flow over a regular array of hills

The flow of water over a regular array of hills in a rotating laboratory experiment is studied as an analogue of planetary boundary layers. Gaussian-shaped hills of heighth = 1 cm andh = 1/3 cm covered the floor of a 228 cm diam rotating tank. The characteristic depth of the Ekman layer varied fromD = 0.1 cm toD = 0.3 cm and the Reynolds number for the Ekman layer varied up toRe = 150. The integrated boundary-layer stress as a function ofRe andD/h was determined by the rate of spin-up of the water after a small increase of the rotation rate.Selected functional relations were fitted to the empirical data by the method of least squares for the laminar and turbulent regimes of flow. These results indicate maximum non-linear effects of the hills and a minimum transition Reynolds number nearD/h = 0.2. The patterns of flow as indicated by dye tracers are briefly discussed. Peculiar circular bands of dye appear to be caused by the selective ejection of dye from the boundary layer when the circular flow is at a particular angle of incidence with respect to the triangular pattern of hills.The research reported herein was supported in part by the National Science Foundation under Grant NSF GA 4388.     

Determination of the Hydrogeologic Properties of Lakebeds Using Offshore Geophysical Surveys

Studies of the relationships between ground-water systems and surface-water systems (lakes) generally require knowledge of the hydrogeologic properties of lakebed materials. Direct measurement of these properties may be prohibitively expensive or difficult in an offshore environment. A correlation between longitudinal conductance, a geoelectric parameter, and lakebed leakance (vertical hydraulic conductivity divided by thickness) may provide a rapid and inexpensive method for estimating lakebed hydrogeological properties using data collected by offshore seismic and electrical surveys. A test of the method at three study sites in Lake Michigan shows a positive linear relationship between the logarithm of longitudinal conductance and the logarithm of lakebed leakance at the three sites.     

Cladding influence on dynamics response of tall buildings

Precast concrete panels form attractive facades for steel frame buildings and are generally regarded as non-structural by structural engineers. However, panels have been found to add lateral stiffness until their capacity or that of their connections is exceeded. Consequently, the computed dynamic response based on a model of the structural framing alone may be quite different from that experienced by the actual structure. As a case study, the influence of precast concrete panels on lateral and torsional stiffness of a 25-storey building was investigated. The effect of cladding on dynamic properties and linear seismic response was explored by varying panel stiffness. Cladding stiffness was added to the bare frame model until analytical frequency values matched vibration test results. Then, using the cladding stiffness values obtained, an accidental eccentricity between centres of mass and rigidity at each floor level was imposed and linear seismic response computed. Torsional response effects were increased substantially. Finally, a modified cladding panel connection was developed based on previously-reported studies for panelized construction. The influence of the proposed connection on overall structural response was determined for different ground motion inputs.     

A reappraisal of the Rb-Sr systematics of early Archaean gneisses from Hebron,Labrador

A re-investigation of the Rb-Sr isotope systematics of early Archaean granodiorite gneisses from Hebron, northern Labrador, confirms the existence of early Archaean crust in that area, but has failed to corroborate both the high degree of coherence and the high initial⁸⁷Sr/⁸⁶Sr reported by Barton [1]. Instead, marked geological scatter is observed in the data for both slabbed gneisses and large bulk samples, a scatter unequivocally due to the development of secondary whole-rock isochrons at ～ 1800 Ma.Regression analysis of the new data gives very large uncertainties in the age and initial⁸⁷Sr/⁸⁶Sr because of this secondary disturbance, viz. 3645_?470⁺⁸⁷⁵ Ma and 0.702_+0.005^?0.009. We interpret the previous indications of high initial⁸⁷Sr/⁸⁶Sr at 3600 Ma as due to chance.The Hebron data cannot be distinguished from similar populations of geologically-disturbed Rb-Sr results from the Uivak I gneisses to the north [5] and the Amîtsoq grey gneisses [6]. This supports a field-based contention that the so-called “Hebron gneisses” are both lithologic and stratigraphic equivalents of the Uivak I gneisses.     

A model for nonlinear wave interactions

Phenomena associated with weakly nonlinear waves are encountered in optics, plasma physics, and geophysics. Specific applications in oceanography include ocean surface waves, internal gravity waves, Rossby waves, and interactions between these wave systems. Techniques for studying these phenomena include direct numerical integration, Boltzmann-like transport equations, and Langevin transient response functions. A simple model, called thetest wave model, is described and used to illustrate comparison of these methods.